Single-wall carbon nanotubes (SWNTs) were discovered in 1993 in soot produced in an arc discharge in the presence of transition metal catalysts. See Iijima et al., “Single-Shell Carbon Nanotubes of 1 nm Diameter,” Nature, 363, pp. 603–605 (1993); Bethune et al., “Cobalt Catalyzed Growth of Carbon Nanotubes with Single Atomic Layer Walls,” Nature, 363, 605 (1993). Such SWNTs, comprised of a single tube of carbon atoms, are the smallest of the carbon nanotubes. SWNTs can have lengths of up to several micrometers and diameters of approximately 0.5 nm–10.0 nm [Saito et al., Physical Properties of Carbon Nanotubes, London: Imperial College Press, 1998; Sun et al., Nature, 403: 384 (2000)], although most have diameters of less than 2 nm (Saito et al.). Diameters as small as 0.4 nm have been reported, but these were formed inside either multi-wall carbon nanotubes (MWNTs) [Qin et al., Chem. Phys. Lett., 349: 389–393 (2001)] or zeolites [Wang et al., Nature, 408: 50–51 (2000)]. SWNTs, and carbon nanotubes of all types have since been produced by other techniques which include gas-phase reaction methods [Hafner et al., Chem. Phys. Lett., 296: 195–202 (1998); Cheng et al., Chem. Phys. Lett., 289: 602–610 (1998); Nikolaev et al., Chem. Phys. Lett., 313: 91–97 (1999)], laser ablation techniques [Thess et al., Science, 273: 483–487 (1996)], and flame synthesis [Vander Wal et al., J. Phys. Chem. B., 105(42): 10249–10256 (2001)].
Single-wall carbon nanotubes (SWNTs) have unique structural, electronic and mechanical properties that make them appealing for a variety of applications [Ijima et al., Nature, 363, pp. 603–605, 1993; Lansbury et al., J. Am. Chem. Soc., 120, pp. 603–604, 1998; Yu et al., Phys. Rev. Lett., 84, pp. 5552–5555, 2000; Baughman et al., Science, 297, pp. 787–792, 2002; Odom et al., J. Phys. Chem. B, 104, pp. 2794–2809, 2000; Kong et al., Science, 287, pp. 622–625, 1998; Rao et al., Chem. Phys. Chem., 2, pp. 78–105, 2001; Gao et al., Adv. Mater., 13, pp. 1770–1773, 2001]. For instance, the remarkable tensile strength of SWNTs has led to the fabrication of a variety of nanotube-reinforced fibers and composite materials [Calvert, Nature, 399, pp. 210–211, 1999; Gong et al., Chem. Mater., 12, pp. 1049–1052, 2000; Yudasaka et al., Appl. Phys. A, 71, pp. 449–451, 2000; Vigolo et al., Science, 290, pp. 1331–1334, 2000; Coleman et al., Adv. Mater., 12, pp. 213–216, 2000; Kumar et al., Macromolecules, 35, pp. 9039–9043, 2002].
Determining the average length of a sample of single-walled carbon nanotubes (SWNTs) is becoming particularly important as the various methods for mass-producing SWNTs are being scaled-up and optimized, and as new methods for chemical and physical cutting are being developed for making SWNTs of prescribed length from the samples produced in the reactors.
Currently, length determination relies chiefly on atomic force microscopy (AFM) measurements, which are time-consuming and suffer the drawbacks of small sample sizes (typically only hundreds of SWNTs) and of possible errors induced by the sample preparation technique (aggregation, preferential adsorption of long or short SWNTs, etc.). Consequently, questions often arise as to how representative the imaged tubes are of a bulk sample. Light scattering has been recently proposed as an alternative method for measuring SWNT length, but this has not yet been proven. Thus, a convenient method for easily and accurately making average length determinations on macroscopic (i.e., bulk) samples of SWNTs would be of great benefit.